Extensometer



Feb. 27, 1951 D. s. woon 2,543,429

EXTENSOMETER Filed June 20, 1949 INVENTOR.

` ATTORNEYS.

Patented Feb. 27, 1951 UNHTED STATES PATENT OFFICE EXTENSOMETER David S.Wood, Sierra Madre, Calif.

Application June 20, 1949, Serial No. 100,189

Claims. (Cl. 33-149) My invention relates to extensometers; that is, todevices intended to measure the extension or contraction of a testspecimen mounted between the relatively movable jaws of a testingmachine, and among the objects of my invention are:

First, to provide an extensometer wherein a cemented wire strain gaugeis incorporated with the extensometer and forms the sensitive elementthereof.

Second, to provide an extensometer of this class which is capable ofmeasuring displacements greater than is possible by use of a cementedwire strain gauge mounted directly on the test specimen.

Third, to provide an extensometer which may be readily and quicklymounted in place and which is not likely to be damaged in the event offailure of the test specimen.

With the above and other objects in View as may appear hereinafter,reference is directed to the accompanying drawings in which:

Figure 1 is a partial sectional, partial top view of my extensometershown in place on the specimen; the View being taken along the line ofI-l of Figure 2.

Figure 2 is a partial elevational, partial sectional view thereof takensubstantially along the line 2 2 of Figure 1.

Figure 3 is an end view thereof.

Figure 4 is a sectional view thereof taken through l-l of Figure 2.

My extensometer comprises two substantially identical units A and Badapted to be mounted on opposite sides of a test specimen S.

Each unit includes a bodyl on two sides of Which are mounted plates 2.The plates project past the inner or specimen side of the body l to formtherewith a channel 3.

One end of the body l, the upper end as shown in the drawings, isprovided with a slot midway between the plates 2. The Slot is traversedby a pin ll which pivots a lever member 5 which is substantiallyL-shaped. One leg 5a of the lever 5 extends in this slot toward the mainportion of the body l, and receives a-set screw B. The set screw bearsagainst a shoulder Ia forming a part of the body I and projecting intothe slot under the leg 5e of the lever. A leaf spring 1 is secured tothe body member between the plates 2 and extends upwardly to bearagainst the lever 5, in a direction tending to urge the leg 5a and setscrew away from the shoulder la.

The remaining leg 5b of the lever 5 extends toward the specimen andterminates in a knife edge extremity 8 which bears in a groove Cprovided in the specimen S. This leg of the lever is also 2 provided onopposite sides with a pair of knife edges 9 pointing in oppositedirection from the knife edge 8.

A beam member Hl is centered in the channel 3. The beam is in the formof a flat strip capable of substantial ilexure. At one end the beam isconnected integrally to a head l i. The extremity of the beam memberremote from the head I l is bifurcated as indicated by Ille,r straddlesthe leg 5b of the lever 5 and bears against the knife edges 9.

The head is provided with opposed knife edges 2 and i3 adapted to bearrespectively in a second groove D in the specimen and in a seat lbprovided in the body member near its end remote from the lever 5.

Secured to one or both sides of the beam l@ is a cemented wire straingauge l 5 which is indicated diagrammatically in Figures 2 and 4. Thestrain gauge per se is conventional and comprises a resistance wirearranged in grid form, with parallel portions either transversely orlongitudinally disposed relative to the beam I0. The wire is cemented tothe beam lil in such a manner that flexure of the beam causes change inthe resistance of the wire. Measurement of this change in resistance ismade to determine the extent of flexure of the beam. The ends of thewire element are connected to suitable terminals l5 provided in the bodyl.

The two extensometer units A and B are provided with laterally directedlugs Il located approximately midway between the upper and lowerextremities of the units.

A U-strap or yoke I8 straddles the two units. Near the cross portion ofthe U-strap, the legs thereof are provided with slots i9 which receivethe lugs Il' of one of the units. The extremities of the U-strap legsare provided with notches 2E) which t over the lugs il of the other unitso that the two units may be held in parallel relation. A set screw 2|is provided in the cross portion of the U-strap i8 so as to bear againstthe adjacent extensometer unit.

Operation of my device is as follows:

The specimen S is prepared in the usual man-- ner; that is, theextremities (not shown) are constructed so as to fitin the jaws of atesting machine, the central portion of the specimen is reduced insection. Just beyond the reduced portion of the specimen the notches Cand D are cut. The spacing of these notches is such that the knife edges8 of the two units may nt into the upper notch C and the knife edges $2may lit into the lower notch D.

The set screw S is employed to pivot the lever so as to move thecorresponding knife edge 8 to or from the knife edge I2. The yoke or U-strap i8 is employed to hold the extensometer units in place and the setscrew 2| is tightened so that the knife edges bear with appropriatepressure in their respective grooves.

The several strain gauges are connected to an electronic device (notshown) capable of measuring changes in resistance of the sensitiveelements contained in the strain gauges. The strain gauges may be soarranged that their changes in resistances are cumulative. In thepre-adjustment of the extensometer the beams ID may be free of strain ormay have a pre-determined initial strain; this being accomplished byadjustment of the set screws 6. Y

Upon attachment of the extensometers, the specimen is subjected towhatever loads it is intended to impose, and the resulting elongationresults in pivotal movement of the head mem.- bers H, and consequentflexure of the beams I which fiexure is measured by the strain gaugesl5. The resulting changes in resistances may be correlated with thechange in dimension of the specimen. It should be pointed out that thecemented wire type of strain gauge shown is instantly responsive tochanges in dimension of the specimen so that even transient loadsimposed on the specimen may be detected and recorded or measured.

If the specimen is to be subjected to compression loads, the knife edgesare initially adjusted so that the beams l0 are flexed beforecompression is applied. Then when the specimen is subjected tocompression, the beams lil straighten.

Having thus described certain embodiments and applications of myinvention, I do not desire to be limited, but intend to claim allnovelty inherent in the appended claims.

I claim:

1. An extensometer unit, involving: a beamv member including a flexibleblade and a head at one end, said head having opposed knife edgebearings, one bearing adapted to engage a specimen; a body structurepivotally engaged by the other knife edge of said head; a leverpivotally mounted in said body member and including a leg terminating ina first knife edge bearing engageable with said specimen at a pointspaced from the knife edge bearing of said head; said lever having asecond knife edge bearing facing in the opposite direction therefrom theextremity of said flexible blade remote from said head and overlyingsaid second knife edge bearing; and cemented wire strain gauges mountedon said blade.

2. An extensometer, involving: a pair of hinged members one arranged tobear against a specimen; a beam structure including a flexible bladebearing against said one member, and a head interposed between the otherof said members and said specimen and pivotally connected with saidother member whereby change in longitudinal dimension of said specimencauses fiexure of said blades; and cemented wire strain gauges mountedon said blades to reflect the degree of flexure thereof.

3. An extensometer unit, involving: a beam including a flexible bladeand a head member having opposed bearing elements disposed in a planenormal to said blade; a lever disposed adjacent the extremity of saidblade and having opposed bearing elements, one of said bearing elementsbeing engageable by the extended portion of said blade; a body structurejournalling said lever and engaging one of the bearing elements of saidhead member; the other bearing elements of said lever and head adaptedto engage a test specimen at spaced points and move longitudinallytherewith causing relative pivotal movement of said lever and body andflexure of said blade; and cemented wire strain gauges mounted on saidblade.

4. An extensometer unit, involving: a body member, a lever pivotallyconnected to said body member and having an arm terminating in a bearingengageable with a test specimen; a beam member including a head disposedin approximate parallelism with said arm and having bearings engageablewith said specimen and said body, and a flexible blade rigidly connectedto said head and extending therefrom to said arm in approximateparallelism with said specimen; said arm having a second bearingsupporting the extremity ofV said beam; and strain gauges secured tosaid beam, said head adapted to tilt on change in the longitudinaldimension of said specimen to cause flexure of said beam.

5. An extensometer, involving: a body member; a lever pivotallyconnected to said body member and including an arm extending atapproximately right angles thereto and terminating in a knife edgebearing facing away from said body member, said arm also havingY asecond knife edge bearing facing toward said body member; a beam memberincluding a head having a irstV knife edge bearing facing away from saidbody member anda second knife edge bearing engagingsaid body member, anda flexible blade, extending from said head in substantial parallelismwithv said body member with its extremity resting against the secondbearing of said arm; and strain gauge means on said blade, s aid firstknife edge bearings of said arm and said head adapted to engageaspeciman and on change in length of said specimen to cause exure ofsaid blade. c,

DAVID S. WOOD.

REFERENCES C'I'ED v The following references are of record in the fileof this patent:

UNITED STATES PATENTS Number Name Date 2,416,664 Ruge Feb. 25, 19472,484,164 Hathaway Oct. 11, 1949 FOREIGN PATENTS- Number., Country Date147,987 Great Britain July 23, 1920 OTHER REFERENCES Publ.: MetalProgress, Nov. 1948, pagesf

